BACKGROUND Calcitriol (1 alpha, 25-dihydroxy vitamin D3) is a good vitamin D supplement but can cause hypercalcemia. Whereas, 22-oxa-1 alpha, 25-dihydroxy vitamin D3 (22-oxa-calcitriol) has less hypercalcemic activity than calcitriol and is reported to be effective for cell-proliferative diseases. The objective of the study was to compare renal function and blood tests of arthritis patients receiving calcitriol supplements with those receiving 22-oxa-calcitriol supplements. MATERIAL AND METHODS A total of 369 patients with clinically confirmed rheumatoid arthritis were included in this phase II trial. Patients received lactose powder (the placebo group, n=123), 50 000 IU/week of 22-oxa-calcitriol (the treatment group, n=123), or 50 000 IU/week of calcitriol (the control group, n=123) for 6 weeks. At the time of enrollment and after 6 weeks of supplementation, renal function tests, blood tests, and secondary outcome measures were evaluated. One-way ANOVA and the chi-squared test for independence were performed for continuous data and constant data at a 95% of confidence level. RESULTS Both 22-oxa-calcitriol and calcitriol successfully decreased swollen joints in patients with rheumatoid arthritis, and both improved Health Assessment Questionnaire Disease Activity Index scores and serum vitamin D levels. The intensity of improvement of serum vitamin D levels in both groups was the same (P<0.0001, q=0.24); however, calcitriol caused hypercalcemia (P<0.0001, q=12.59). CONCLUSIONS This study found that 22-oxa-calcitriol was a good option for vitamin D supplementation in rheumatoid arthritis patients.